Great starting points for all. I think this is the best place to start for any newbies…then narrow down by getting insider info, etc on CC. It is hard for CC posters to recommend specifics, when you yourself haven’t figured out what you really want in a boarding school.
@Nantar you might take a look at Millbrook. It meets your requirements. One of my sons is extremely bright (very high test scores) but was learning to deal with his ADHD, and needed an environment that wasn’t high stress/pressure to the max, as his adhd added enough difficulty. He was able to thrive and flourish there. (And ended up at a T20 university).